1. Field of the Invention
The present invention relates generally to reconstitutable fuel assemblies for nuclear reactors and, more particularly, is concerned with a removable and reusable locking pin for permitting assembly and disassembly of upper holddown and lower adapter plates of a removable top nozzle subassembly of a reconstitutable fuel assembly.
2. Description of the Prior Art
Conventional designs of fuel assemblies include a multiplicity of fuel rods held in an organized array by grids spaced along the fuel assembly length. The grids are attached to a plurality of control rod guide thimbles. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the control rod guide thimbles which extend above and below the opposite ends of the fuel rods. At the top end of the fuel assembly, the guide thimbles are attached in openings provided in the top nozzle. Conventional fuel assemblies also have employed a fuel assembly hold-down device to prevent the force of the upward coolant flow from lifting a fuel assembly into damaging contact with the upper core support plate of the reactor, while allowing for changes in fuel assembly length due to core induced thermal expansion and the like. Such hold-down devices have included the use of springs surrounding the guide thimbles, such as seen in U.S. Pat. Nos. 3,770,583 and 3,814,667 to Klumb et al and 4,269,661 to Kmonk et al, and in the first patent application cross-referenced above.
Due to occasional failure of some fuel rods during normal reactor operation and in view of the high costs associated with replacing fuel assemblies containing failed fuel rods, the trend is currently toward making fuel assemblies reconstitutable in order to minimize operating and maintenance expenses. Conventional reconstitutable fuel assemblies incorporate design features arranged to permit the removal and replacement of individual failed fuel rods. Reconstitution has been made possible by providing a fuel assembly with a removable top nozzle. The top nozzle is mechanically fastened usually by a threaded arrangement to the upper end of each control rod guide thimble, and the top nozzle can be removed remotely from an irradiated fuel assembly while it is still submerged in a neutron-absorbing liquid. Once removal and replacement of the failed fuel rods have been carried out on the irradiated fuel assembly submerged at a work station and after the top nozzle has been remounted on the guide thimbles of the fuel assembly, the reconstituted assembly can then be reinserted into the reactor core and used until the end of its useful life.
One recently proposed design for a reconstitutable fuel assembly top nozzle is described and illustrated in the second patent application cross-reference above. The proposed top nozzle overcomes certain problems associated with the removable top nozzle designs of the above-cited U.S. patents and patent application. Specifically, it includes improved structures which eliminate relative moving contact or sliding engagement between the upper core support plate and the hold-down structure of the top nozzle while providing removable mounting of the top nozzle as a unitary subassembly on the guide thimbles of the reconstitutable fuel assembly as well as desired alignment of the fuel assembly with the upper core plate. Relative motion between the upper core plate and alignment sleeves of the top nozzle still takes place but without damaging contact with one another. Also, any deleterious effects on the hold-down springs from coolant cross-flow between fuel assemblies are substantially eliminated in the proposed top nozzle by the incorporation of an enclosure wall about the perimeter of the top nozzle which protects the springs from flow induced vibration.
The proposed top nozzle includes an upper hold-down plate which compresses a plurality of hold-down springs, preferably four in number, against a lower adapter plate. Lugs associated with the upper hold-down plate and the enclosure associated with the lower adapter plate are slidable coupled together via locking shear pins which restrict their relative movement away from each other but allow movement toward each other. Elongated alignment sleeves at their upper ends extend through passageways in the upper hold-down plate and at their lower ends are releasably threaded to the upper ends of the guide thimbles which extend upwardly through holes in the lower adapter plate. In such arrangement, when the lower ends of the alignment sleeves are tightened against the adapter plate, they cooperate with lower retainers on the guide thimbles below the adapter plate to mount the top nozzle via the adapter plate on the guide thimbles. Conversely, for removal of the top nozzle, the alignment sleeves are unthreaded from the guide thimbles. The sleeves remain captured between the upper hold-down and lower adapter plates, as also do the respective hold-down springs which encompass the sleeves, after the sleeves are threadably disconnected from the guide thimbles and the top nozzle is removed as a unitary subassembly from the fuel assembly.
While the proposed removable top nozzle design of the second cross-referenced patent application, as just briefly described, has proven to be a highly satisfactory solution to the problems previously existing and permits assembly and disassembly of the top nozzle as a unit upon and from the guide thimbles of the fuel assembly, it makes no provision for disassembly of the top nozzle itself. During initial fabrication and later during reconstitution of the fuel assembly, it would be advantageous to be able to disassemble the top nozzle to allow access to the hold-down springs and the alignment sleeves in the event that replacement of one or more of these components is required. As briefly mentioned above, the upper hold-down and lower adapter plates of the top nozzle are slidably connected together by four locking shear pins. These shear pins are generally welded in place and are only considered to be removable in the limited sense that in the process of forceable removal they are ordinarily destroyed. Also, some damage will likely be incurred by the lower adapter plate enclosure to which these shear pins were welded, which must be repaired before new shear pins can be installed.
Consequently, a need exists for a different approach to locking the upper hold-down plate and lower adapter plate of the top nozzle in their slidable connection, one which will allow relatively easy disconnecting or disassembling of the respective top nozzle plates while reliably maintaining the plates in their slidable connection when assembled together.